Sources of sulfur in the Nandong underground river system, southwest China: A chemical and isotopic reconnaissance

Abstract

Abstract Sulfur isotopic compositions (δ34S) and major ions were measured in groundwater and surface waters in order to elucidate the source(s) of elevated SO 4 2 - content in some of these waters in the Nandong Underground River System (NURS), located in an agricultural and urbanization-dominated area in SW China. The major chemical composition of analyzed groundwater and surface water from NURS was characterized by Ca–HCO3 or Ca (Mg)–HCO3 type in forested and grass land areas, and Ca–Cl (+NO3 + SO4) or Ca (Mg)–Cl (+NO3 + SO4) type in urban and agricultural areas, respectively. The anomalously high SO 4 2 - concentrations in waters collected from urban and agricultural areas indicated a likely anthropogenic origin for SO 4 2 - . The groundwater collected from forested and grass land areas was characterized by relatively low δ 34 S – SO 4 2 - , from −1.7‰ to +4.3‰, and low SO 4 2 - concentrations of less than 10 mg L−1, consistent with an origin for SO 4 2 - in these waters from a mixture of rainfall-derived SO 4 2 - and SO 4 2 - from oxidation of bedrock sulfide. High SO 4 2 - groundwater and surface water (>20 mg L−1) collected from urban areas was characterized by elevated δ 34 S – SO 4 2 - , from +8.3‰ to +12.1‰, compared to lower δ 34 S – SO 4 2 - in samples from cultivated land areas (−3.7‰ to +6.4‰), indicating that the SO 4 2 - in these two groups of waters resulted from different anthropogenic sources. The S isotopic composition of SO 4 2 - in urban areas is consistent with addition of SO 4 2 - from detergent/sewage, whilst in cultivated land areas isotope composition indicates that additional SO 4 2 - could be derived from fertilizers and/or increased oxidation of sulfide.